The present invention relates to the field of superconductivity. More particularly the invention relates to a buffer layer structure based on doped ceria for providing optimized lattice match with a YBCO layer in a conductor, a lattice matching layer to be included in said buffer layer structure and a process of manufacturing thereof. The solution proposed by the present invention, minimizes the lattice mismatch problems between YBCO and the underlying layer during the deposition process. This makes it possible to obtain relatively higher current density values, thus improving the current transport features of the tape conductor.
Ceria is CeO2 (cerium oxide), which is a known material to be used as a buffer layer in superconducting thin film technology. YBCO stands for Yttrium Barium Copper Oxide with the chemical formula of: YBa2Cu3O7. YBCO is a superconducting ceramic. A superconducting ceramic like YBCO has the property of conducting large DC electrical current densities (up to several MA/cm2 at temperaturesxe2x89xa677K., and in a zero applied magnetic field) with zero resistancexe2x80x94and large AC current densities with negligible resistance, as compared with metals, up to high frequencies (beyond 1 GHz).
As it is known in the related art, a superconductive tape is generally configured as a multilayer structure comprising a bi-axially textured substrate over which or one or more epitaxial buffer layers are deposited. The bi-axially textured substrate can be either a bi-axially textured Ni substrate, as in the RABiTS process [see C. Park, et al., IEEE Transactions Applied Superconductivity Vol.9, no 2 (1999) 2276-2279], or a Ni-based alloy, coated by a bi-axially textured oxide layer, e.g. YSZ (Yttrium stabilized zirconia) bi-axially textured by the IBAD process [Y. Iijima, et al, Applied Physics Letters 60 (1992) 769]. IBAD stands for Ion Beam Assisted Deposition.
The buffer layer itself is actually made of several layers, each layer having a specific role, such as diffusion barrier against Ni, bi-axial texturing when the metallic substrate is a polycrystalline Ni alloy, and lattice matching with the YBCO layer. Some examples of such buffer layers are YSZ and ceria, often used as a lattice matching layer. The YBCO layer is then deposited over said buffer layers. One of the important factors in determining the quality of a superconductive structure is the so called current density, Jc, which is measured as a function of electric current per cm2 at a standard temperature. Current values for Jc measured on YBCO tape conductors are of the order of several 105/cm2 at 77K. Therefore, any improvement in the current density value would improve the current carrying capacity of the tape conductors.
However, in order to obtain higher current density (Jc) values, one is faced with certain problems in the manufacturing process.
One of such problems is the difficulty in achieving bi-axially textured YBCO layers, of at least 1 xcexcm thickness, capable of exhibiting Jc values above 106 A/cm2 at 77K. These properties can only be achieved if firstly the YBCO layer is bi-axially textured and secondly, if the bi-dimensional growth of YBCO is maintained over hundreds of nanometers, which is not the case with current lattice matching layers such as Y2O3, pure CeO2 or YSZ. The optimal bi-axial texture of the YBCO layer is specified by grains with their c-axis perpendicular to the surface of the layer and an in-plane crystalline misorientation between grains which is substantially below 10xc2x0.
A further requirement is that the surface coverage of the texture by grains with an a-axis perpendicular to the film plane should be below 1 percent. This is achieved by adjusting the deposition temperature of the YBCO layer in a narrow window, e.g. centered about 765xc2x0 C. in the case of Pulsed Laser-assisted Deposition [see D. Chambonnet, et al., Physica C 235-240, (1994) 625-626].
In the above requirements, bi-dimensional growth means growth without screw dislocations perpendicular to the surface of the layer. The onset of screw dislocation occurs because of the lattice mismatch between YBCO and the buffer layer, the density of screw dislocations increasing with the lattice mismatch. The lattice mismatch is defined as the relative difference in lattice parameters between two epilayers. In the case of YBCO and ceria, at the YBCO deposition temperature, the lattice parameters of YBCO and ceria are 0.3889 nm and 0.3859 nm respectively. The lattice mismatch is the following ratio: (0.3889xe2x88x920.3859)/0.3889=7.7xc3x9710xe2x88x923.
As soon as screw dislocations appear, due to elastic strain relaxation, grain growth is driven by screw dislocations. This results in a so-called terrace growth morphology, with an increasingly rough surface as the film thickness increases.
Moreover, the size of the metallurgical grains being in close relationship with the density of screw dislocations, this implies that the lateral size of the metallurgical grains decreases as the density of screw dislocations increases. A net consequence is that the crystalline disorder increases at grain boundaries. This results in reduced transport properties at grain boundaries, i.e. lower Jc values. Moreover, the disorder at grain boundaries and the surface roughness favor the penetration of magnetic vortices, which are responsible for electrical losses, in the YBCO layer. Thus, Jc becomes very sensitive to self or external applied magnetic fields.
As discussed above, the buffer layers conventionally used are YSZ or CeO2. However, the use of these layers as such gives rise to the drawback that at a typical deposition temperature, such as for example 765xc2x0 C., the relative mismatch between these materials and YBCO is too large, namely in the range of about 5% for YSZ/YBCO and 0.8% for CeO2/YBCO. In the best of these two cases, the onset of screw dislocations arises at a critical thickness (tc) of around 100 nm, whereas as discussed above, the desired tc value should be in the range of several hundred nanometers in order to grow high quality thick YBCO films.
It is therefore desired to use a buffer layer in a conductor structure such that the relative mismatch produced by said buffer layer at a normal deposition temperature, e.g. at about 765xc2x0 C., is substantially below 0.1%. This lattice mismatch barrier yields an expectation of tc values of several hundred nanometers, e.g. 300 nm for 0.1%.
In order to overcome the above drawbacks, the buffer layer structure based on doped ceria for YBCO tape conductors, a lattice matching layer to be included in said buffer layer and a process of manufacturing thereof, objects of the present invention are proposed.
According to the invention, the use of a dopant, which maintains the fluorite crystalline structure of ceria up to a large volume fraction, can cause a substantial change in the lattice parameter of the compound. The partial substitution of Ce by the dopant (La for instance) results in a compound, which eventually maintains the crystalline symmetry of the host (ceria), up to a large atomic fraction, and exhibits a lattice parameter which varies with the dopant concentration. In the case of La-doped ceria, the lattice parameter linearly varies with the atomic concentration of the dopant (known as the Vegard""s law), up to an atomic fraction of 0.55 [see B. C. Morris, et al., J. Mater. Chem. 10 (1993) 1007]. Several dopants are known to have such property, such as lanthanum (La), calcium (Ca), magnesium (Mg), chromium (Cr), europium (Eu), iron (Fe), hafmium (Hf), manganese (Mn) and neodymium (Nd).
Accordingly, one object of the present invention is that of providing a conductor structure comprising a metallic substrate tape over which there is provided at least one buffer layer comprising a CeO2 layer doped with a dopant, and having a superconductive layer of YBCO on said CeO2 layer characterized in that said CeO2 layer is a lattice matching layer.
According to another aspect of the present invention, the metallic substrate is selected so as to have a thermal expansion coefficient xcex1 close to that of YBCO, namely in the range of xe2x88x92200-800xc2x0 C.
According to still a further aspect of the present invention, the CeO2 lattice matching layer may have a thickness substantially inferior or equal to 100 nm and the YBCO layer may have a thickness substantially superior or equal to 1 xcexcm.
In a preferred embodiment according the invention, the buffer layer comprises between the substrate and the CeO2 lattice matching layer a first layer, being a diffusion barrier against the substrate metal. And this first layer may be a SiNx layer having a thickness comprised between 10 nm and 100 nm.
Furthermore, in this embodiment, the buffer layer comprises between the first layer and the lattice matching layer a second layer which is bi-axially textured for transferring its texture to the YBCO layer at a proper deposition temperature. And this second layer may be a MgO layer, having a thickness substantially inferior or equal to 10 nm.
In one embodiment according the invention, the metallic substrate is a Ni alloy.
In another embodiment according the invention, the metallic substrate is a biaxially textured Ni substrate.
According to an aspect of the present invention the dopant is lanthanum. And said lanthanum may have an atomic fraction substantially equal to 0,15.
Another object of the present invention is that of providing a method of manufacturing a conductor structure previously described characterized in that it comprises:
a depositing step by PVD of the lattice matching layer of CeO2 doped on a substrate layer;
a depositing step of the superconductive layer of YBCO on the CeO2 lattice matching layer.
According to another aspect of the present invention, the method may comprise a depositing step by PECVD or PVD of the first layer on the metallic substrate.
More over, the method may comprise a depositing step by PVD with IBAD of the second layer on the first layer, the substrate layer being the second layer.
These and further features of the present invention are described in more detail herein below, in the description of a preferred embodiment as well as in the claims.